(1) Field of the Invention
The present invention relates to a single-photon generator and a single-photon generation method for use in, for example, quantum cryptography and a quantum information processing field.
(2) Description of Related Art
Toward the realization of a next-generation information society including electronic government and electronic commerce, safe and secure cryptographic communication is inevitable.
At present, the public encryption key system and the secret encryption key system are used for cryptographic communication. In the RSA public encryption key system currently in wide use, security is guaranteed only by a calculation amount side that an enormous time is needed to solve a prime factorization of an extremely large number using a polynomial. Therefore, when a quantum computer having excellent capability of ultra high-speed parallel computation comes on the scene, the time required for decrypting such encrypted information will be drastically shortened, and the security will not be guaranteed any more. In other words, the security of the public encryption key system and the secret encryption key system currently in use is not absolutely perfect.
For example, in case of the public encryption key system, once a public key is decrypted, there is a risk of wiretapped or falsification of data by a third party. Also, in case of the secret encryption key system, an information sender and a receiver own an identical secret key, and data encrypted by the sender using the secret key is decrypted by the receiver using the identical secret key. In such the secret encryption key system, there is a risk that the exact secret key is wiretapped when the secret key is distributed to the two parties concerned.
The quantum cryptography is expected to be a means for solving such a security problem.
A well-known system in the quantum cryptography is the “BB84” protocol proposed by C. H. Bennett and G. Brassard in 1984.
In the above protocol, information is carried on each single-photon for transmission, not on an aggregate of photons as is conventionally used in the optical communication. If one information bit is given to one photon, for example, a polarized state of the photon, it is neither possible to extract nor to duplicate without destroying the photon state, because each photon follows the Heisenberg's uncertainty principle (a principle that conjugate physical quantities cannot be measured simultaneously with accuracy) and the no-cloning theorem (a theorem that no duplication can be made without observing a quantum state).
Accordingly, when duplication (wiretapping) or falsification is performed on a communication path by a third party, it is possible to detect immediately, and only a ‘clean’ key not observed by the third party can be shared by the sender and the receiver. Thus, the security of the encryption key shared by the two parties is guaranteed based on the physical principle, not on difficulty in respect of the computation amount, as long as the carrier of information is a single-photon.
In recent years, commercial use of a quantum cryptography system has been in progress.
In the quantum cryptography system, a sender side includes a single-photon source for generating one photon (single-photon) and a controller of a polarized state or a phase state for adding secret key information to the photon, while a receiver side includes a single-photon detector for detecting the photon information.
As the single-photon source, normally a laser light source and an attenuator are used. In such the single-photon source, a laser pulse stream is output from the laser light source, and the light intensity of the laser pulse stream is attenuated by the attenuator, so that a mean number of photons per pulse becomes one or less. Thus, a single-photon is generated in a simulated manner.
However, when generating the single-photon in such the simulated manner, the generation of a plurality of single-photons cannot completely be reduced to zero. When the plurality of single-photons are included in one pulse, a security problem occurs because wiretapping can be performed by stealing a portion thereof without being noticed by the receiver.
To suppress a generation ratio of the plurality of photons, it is an effective method to increase the attenuation rate of the laser light. However, the above method leads to the cost of a decreased key transmission rate.
Meanwhile, to extend a key transmission distance, it is also important to use a communication wavelength band (1.3-1.55 μm) producing less transmission loss in an optical fiber.
At present, quantum key distribution of 100 km or longer is reported by means of the BB84 protocol using a simulated single-photon of 1.55-μm band. (Refer to “Quantum key distribution over 122 km of standard telecom fiber”, C. Gobby et al., Applied Physics Letters, Vol. 84 No. 19, p. 3762-3764, May 10, 2004.) However, a key distribution speed is 1 Hz or less for 122 km.
For the above reasons, in order to achieve high-speed, long-distance quantum key distribution, a true single-photon generator in a communication wavelength band becomes necessary.
As to such a true single-photon generator in a communication wavelength band, a lot of studies have been carried out so far.
As a well-known method, there has been a method for extracting a photon one-by-one by exciting a carrier in an isolated two-level system through optical pumping or current injection, and using an exclusive recombination process. For example, it is considered to use a single molecule (Refer to “Single photons on demand from a single molecule at room temperature”, B. Lounis et al., Nature, Vol. 407, p. 491-493, Sep. 28, 2000), a nitrogen-vacancy color center in a diamond crystal (Refer to “Stable Solid-State Source of Single Photons” Christian Kurtsiefer et al., Physical Review Letters, Vol. 85, No. 2, p. 290-293, Jul. 10, 2000), a quantum dot [Refer to (1) “Single-Photon Generation in the 1.55-μm Optical-Fiber Band from an InAs/InP Quantum Dot”, Toshiyuki Miyazawa et al., Japanese Journal of Applied Physics, Vol. 44, No. 20, L620-L622, 2005, and (2) “Highly efficient triggered emission of single photons by colloidal CdSe/ZnS nanocrystals”, X. Brokmann et al., Applied Physics Letters, Vol. 85 No. 5, p. 712-714, Aug. 2, 2004], and so on. In particular, a quantum dot has a merit capable of varying a wavelength depending on a material or a size. For a typical quantum dot, since the recombination lifetime of an electron-hole pair is on the order of 1 ns, in principle, the rate of generation of single-photons can be increased to the order of GHz.
Recently, a single-photon generating device of a communication wavelength band using an InAs self-organized quantum dot on InP has been reported (Refer to the aforementioned paper “Single-Photon Generation in the 1.55-μm Optical-Fiber Band from an InAs/InP Quantum Dot”). Meanwhile, operation at room temperature is also reported in the devices using a single molecule (Refer to the aforementioned paper “Single photons on demand from a single molecule at room temperature”), a nitrogen-vacancy color center in a diamond crystal (Refer to the aforementioned paper “Stable Solid-State Source of Single Photons”), and a CdSe quantum dot (Refer to the aforementioned paper “Highly efficient triggered emission of single photons by colloidal CdSe/ZnS nanocrystals”). Also, in addition to the above papers, the Japanese Patent Laid-Open Nos. 2003-249928 and 2000-216775 have been obtained, as a result of the prior art survey.
However, in the above-mentioned devices using a single molecule (Refer to the aforementioned paper “Single photons on demand from a single molecule at room temperature”), a nitrogen-vacancy color center in a diamond crystal (Refer to the aforementioned paper “Stable Solid-State Source of Single Photons”) and a CdSe quantum dot (Refer to the aforementioned paper “Highly efficient triggered emission of single photons by colloidal CdSe/ZnS nanocrystals”), a great loss is produced in an optical fiber because the wavelength region lies in short wavelengths (on the order of 500-600 nm), and therefore, it is difficult to use as a single-photon source for long-distance transmission.
Also, as to the above-mentioned single-photon generating device of the communication wavelength band using an InAs quantum dot (Refer to the aforementioned paper “Single-Photon Generation in the 1.55-μm Optical-Fiber Band from an InAs/InP Quantum Dot”), since it is necessary to maintain a specimen temperature on the order of 10k, a cooling agent such as liquid helium is required, which inevitably causes an overall apparatus to become large in size.